Acute Gastroenteritis (AGE) is significant cause of morbidity in children under 5 years of age in the US causing approximately 10% of hospitalizations in this age group. Rotavirus (RV) is the primary cause of severe AGE, resulting in approximately 40% of all AGE-related healthcare encounters. Recently licensed RV vaccines (RW) demonstrated >90% efficacy and promise to significantly reduce the significant burden of AGE throughout the US. Because efficacy trials may sacrifice external validity (or generalizability) to optimize the internal validity of their findings, post-licensure studies are needed to determine the impact of these new vaccines as they are adopted for general use in a real world setting. Our proposed study aims to determine the incidence of and risk factors for AGE-related healthcare encounters in the era of RW's. Consistent with the mission of NICHD to ensure all children have the chance to achieve their full potential for healthy and productive lives free from disease, our long-term objectives are to maximize efforts to reduce the burden of AGE throughout the US and eventually translate our findings to minimize this disease in children across the world. We will conduct a retrospective cohort study comparing the number of AGE episodes in vaccinated versus unvaccinated children in the Children's Hospital of Philadelphia's (CHOP) large practice-based research network over the first 2 consecutive RV seasons following the introduction of RW. The cohort is estimated to be approximately 60,000 patients. Data to be used are collected by CHOP'S practice-based research network comprised of 33 affiliated outpatient practices. The exposure will be defined as receipt of any dose of RW;the primary outcome will be the number of outpatient AGE-related primary care office visits per RV season. ICD-9-CM AGE discharge codes will be used to identify AGE-related visits. Secondary outcomes will be AGE-related after-hours telephone calls, ED visits and hospitalizations. For the analysis, we will characterize the subjects by all potential risk factors, conduct univariable analysis to determine associations between risk factors and the outcome, and perform a multivariable analysis using Poisson regression. Acute gastroenteritis (AGE) is a significant public health burden in the US with rotavirus being a primary cause. Recently licensed rotavirus vaccine should significantly reduce the burden of AGE in the US. It is important to study the changing epidemiology of the disease to determine if new pathogens emerge and to maximize the effectiveness and the impact of the rotavirus vaccines.